The present invention relates to a belt-type fixing device that fixes a toner image on a recording material by heating and applying pressure to the toner of the toner image, and relates to an image forming apparatus equipped with the belt-type fixing device.
In an image forming apparatus of an electrophotographic type such as a copying machine, a printer, a facsimile machine and a multifunctional machine equipped with the aforesaid various functions, a latent image corresponding to a document is formed on a photoconductor, then, the latent image is supplied with toner to become a visible image, and this visible toner image is transferred onto a recording sheet, and after this, the toner image transferred onto the recording sheet is fixed, and the recording sheet is ejected.
When forming a color image, latent images for Y, M, C and K corresponding to a document are formed respectively on four photoconductor drums, then, visualized four toner images are transferred primarily onto an intermediate transfer body composed of an endless belt, and they are transferred secondarily onto a recording sheet, thus, the toner image transferred onto the recording sheet is fixed and the recording sheet is ejected.
As a fixing device that fixes a toner image in the aforesaid way, there is a fixing device of a belt type wherein there are provided an endless fixing belt wound around a heat roller having therein a heating device such as a halogen heater and further around a pressure roller, and an outer pressure roller that presses the pressure roller through the fixing belt, and wherein the recording sheet onto which the toner image has been transferred is heated and pressed in a nip portion formed by the fixing belt and the outer pressure roller while the recording sheet is interposed and conveyed. A belt-type fixing device of this kind has merit in that the warm-up time is shortened, resulting in energy savings because the fixing belt of a belt-type fixing device of this type has a small heat capacity.
For making fixing at a higher speed possible, in the belt-type fixing device of this kind, it is necessary to secure a great width of a nip portion. Further, for improving an efficiency of separation from a fixing belt, it is necessary to secure a great curvature on a separating portion on the fixing belt. For both of the foregoing matters mentioned above, it is effective to make an elastic layer of a pressure roller to be lower in hardness.
Though low hardness can be obtained easily in the case of sponge, when it is used for the elastic layer of the pressure roller, pressure in a nip becomes uneven and fine uneven gloss is generated in an image to lower image quality, because of uneven density in sponge. In addition, durability of a roller employing sponge is deteriorated.
Solid rubber gives uniform pressure, and it is free from the image quality decline and durability shortage. However, when rubber of low hardness is used, there is caused a problem of a tack property, resulting in high close adhesion and a state similar to adhesion. In the nip portion, a solid rubber layer is squeezed by high load, and a surface of the solid rubber shrinks once before entering the nip portion, and then is extended gradually as the pressure in the nip portion increases, and after that, a progress opposite to the foregoing is taken after passing the maximum pressure area.
On the other hand, a belt basic body is made of a metal like nickel electroformed metal and a high strength heat-resistant resin such as polyimide, and its elastic modulus is greater than that of rubber, and it hardly shows expansion and contraction. Therefore, it is necessary for the inner surface of the fixing belt to slide on the surface of the pressure roller in the nip portion. However, if a tack property is high, both of them cannot slide easily on each other and troubles that they are forced to slide to generate abnormal noises are generated, or the fixing belt yields and wrinkles are caused thereon.
To avoid the foregoing, it is also possible to grind a solid rubber surface of the pressure roller to acquire an appropriate roughness, and thereby, to make it slide easily. However, in the case of low hardness rubber, the rubber easily becomes a large lump when it is torn off by a grindstone, and thereby, the condition for grinding needs to be controlled extremely strictly for obtaining appropriated roughness, and polishing powder and a portion of rubber immediately before being torn off tend to stay on the surface. After being mounted in a fixing device, uneven pressure distribution is caused in the nip portion by the polishing powder and broken pieces. Further, the polishing powder and others are transferred to a fixing belt and a heat roller, to make heat conduction from the heat roller to be uneven, resulting in uneven temperature of the fixing belt. This unevenness in pressure and temperature results in image defect such as uneven gloss. Further, since the surface is still composed of a rubber of low hardness, tack property to some extent still remains, and even when a fixing belt and a pressure roller can be driven, a state where the pressure roller clings to the inner surface of the fixing belt is generated, in the course of assembling a fixing device, resulting in extremely degraded easiness of assembling. In addition, if they are forced to be assembled, the fixing belt is easily damaged.
Therefore, for preventing the tack property on the surface of the pressure roller, it can be considered that the surface of solid rubber provided on the pressure roller is covered or is coated by a tube of PFA (perfluoroalkoxy) or of PTFE (polytetrafluoroethylene) representing resin with low friction. In this structure, sliding property on a fixing belt is improved by a resin layer.
A fixing device wherein a silicone rubber layer of the pressure roller is covered with a PFA tube is known (see Unexamined Japanese Patent Application Publication No. 2004-94079).
A surface roughness corresponding to the use is needed for the surface of an elastic roller used for a copying machine or the like, for securing friction against a recording sheet. However, when the elastic roller is made to be small, lowering the hardness of the elastic layer is necessary, because an elastic layer also becomes thinner. A patent gazette discloses an elastic roller wherein plural fine grooves are formed in the axial direction when producing the elastic roller of this kind, because forming evenly fine undulations on the surface through conventional grinding processing or electrical discharging machining is not possible. Incidentally, forming of the fine grooves is conducted through injection molding employing a mold that is formed from a mother mold made by grinding processing by a grindstone (see Unexamined Japanese Patent Application Publication No. 10-156841).
Further, the patent gazette discloses a thermal fixing roller wherein highly accurate geometric undulations are formed on the surface of an elastic layer for preventing a sheet from coiling (see Unexamined Japanese Patent Application Publication No. 5-53467).
When a resin layer made of PFA or PTFE is formed on the surface of a pressure roller, easiness of sliding on a fixing belt is improved, and no abnormality is caused on rotation of the fixing belt, thereby, neither wrinkles on the recording sheet nor abnormal noises are generated, and a problem that a work efficiency for assembling of a fixing device is lowered is not caused.
However, when a pressure is applied in the nip portion as stated above, a low hardness solid rubber of the pressure roller is crushed, and the surface tends to extend or contract. For obtaining a wide nip portion or a high separation curvature, the solid rubber is transformed greatly, and the elasticity rate without a resin layer becomes to be about ±10%. In the initial state, the resin layer disturbs this transformation and supports, but it cannot support for a long time, because a resin layer is formed as extremely thin as 10-50 μm so that tenderness of the pressure roller may not be deteriorated. Since the resin layer does not have elasticity like that of rubber, the resin layer is gradually expanded while it repeats its passage through the nip portion, and cracks are generated when being expanded. Since there is no support by a resin layer on the portion of the cracks, transformation is concentrated on the rubber layer of that portion, and cracks progress not only on the resin layer but also on the rubber layer. Under such condition, fixing pressure is not applied on the portion of cracks sufficiently, and uneven gloss and fixing troubles are caused. Further it results in damages of the rubber layer in the worst case.
The invention has been achieved in view of the aforesaid problems, and its objective is to propose a belt-type fixing device wherein cracks affecting adversely a resin layer and an elastic layer by pressure in the nip portion are not generated even when an elastic layer of the pressure roller in the belt-type fixing device is covered by a resin layer, and to propose an image forming apparatus equipped with the belt-type fixing device.
Incidentally, Unexamined Japanese Patent Application Publication No. 2004-94079 does not disclose the problem that a PFA tube and a silicone rubber layer are torn, and countermeasures for the problem of this kind is not described at all.
In Unexamined Japanese Patent Application Publication No. 10-156841, only an elastic layer is formed on the outer circumferential surface of a core material in the elastic roller, and fine grooves are formed for causing surface roughness of the elastic layer to be uniform. Therefore, the structure to cover the elastic layer further with a resin layer is not described, and the problems that cracks are caused on the elastic layer and countermeasures for the problems are not described at all.
In the case of Unexamined Japanese Patent Application Publication No. 5-53467, a fixing heat roller is one wherein a geometric undulation forms are provided on the surface of the elastic layer on the core metal member, and the structure for covering the elastic layer further with a resin layer is not described, and it is one for preventing a sheet from clinging to the fixing roller, thus, the problems that cracks are generated on the elastic layer and countermeasures for the problems are not described at all.